Electrical circuits typically include a distribution panel that divides the electrical power supply into branch circuits protected by commonly known circuit breakers, which interrupt the circuit for overcurrent and short-circuit faults. Branch circuits near water-prone areas are protected by commonly known ground fault circuit interrupters that measure the current imbalance between the line conductor and the neutral conductor caused by current leakage to ground. Branch circuits are also protected from surges in the electrical power supply or from lightning strikes by commonly known surge protectors.
Arcing in an electrical circuit is a fault condition where an intermittent flow of current occurs along or across conductors or to ground causing a discharge of heat energy that builds up in time until the arcing burns the wire insulation, and then, burns the surrounding materials to cause an electrically generated fire. Arcing occurs when the electric field between two or more separated conductors reaches the ionization potential of the separating medium, which is often the gap between conductors. Arcing can result from corroded, worn and loose connections, deteriorated insulation of conductors, construction staples incorrectly applied through conductors, improper post-construction alterations, nails and the like unintentionally driven through concealed conductors in a wall, and even more problematic, conductors damaged by overloading from the electrical power supply before the distribution panel circuit breaker trips.
According to the National Fire Protection Association (NFPA) in 2010, U.S. fire departments responded to an estimated annual average of 50,900 reported home/dwelling structure fires involving electrical failure or malfunction as a factor contributing to ignition over the 2003-2007 time period. These electrical fires resulted in an estimated annual average of 490 civilian deaths, 1,440 civilian injuries and $1.3 billion in direct property damage. The NFPA further states that electrical circuit arcing appears to account for most home/dwelling electrical fires.
To prevent property fires and save lives, electrical circuits need to be protected from arc faults due to line-to-line, line-to-neutral and line-to-ground conductivity, known as high current parallel arcing, and protected from arc faults occurring along line-to-line, line-to-load, load-to-load, load-to-neutral and neutral-to-neutral conductor configurations, known as low current series arcing. A device that protects electrical circuits from these arc faults is commonly known as a combination-type arc fault circuit interrupter (AFCI), which opens the circuit when parallel or series arc faults are detected. While prior art AFCI designs use separate methods and techniques for parallel and series arc fault detection, the present AFCI invention detects parallel and series arc faults with a microprocessor-executed integrated method using time-domain algorithms.
The present AFCI invention could be manufactured as a housing device in the form of a circuit breaker, a receptacle, a receptacle outlet, a cord attachment plug, a portable multiple outlet strip, or could be integrated into another electrical device or system for single or multiple-phase AC or DC applications. Other commonly known electrical circuit faults that include overcurrent, short-circuit, surge and ground leakage could also be detected as part of the present AFCI invention.
According to the 2008 National Electric Code, a listed combination-type AFCI is required to protect 120 volt, single-phase AC, 15/20 amp home/dwelling electrical circuit outlets from parallel and series arc faults. To be commercially available for sale, AFCI devices must meet the UL 1699 AFCI standard qualification requirements. Prior art descriptions of series arc fault detection at low current load levels are limited to operating at the UL 1699 AFCI standard minimum requirement of 5 amps. Unlike prior art AFCI designs that meet the UL 1699 standard requirement for series arc fault detection down to 5 amps, the present AFCI invention is a system employing a bi-directional Hall-effect current sensor (HECS) integrated circuit, a HECS measurement phase shift correction circuit, and among other components, a microprocessor with an integrated method to detect high current parallel arcing and low current series arcing down to 0.5 amp or lower as any specific design might require using time-domain algorithms.
Detecting series arc faults at low current load levels is an improvement over prior art AFCI designs because home/dwelling bedroom electrical circuits could typically operate at less than the UL 1699 AFCI standard minimum requirement of 5 amps, and thus, those series arc faults at low current load levels will likely not be detected by prior art AFCI designs and electrical fires could result. The importance of series arc fault detection at low current load levels is even more significant with the enacted Energy Independence and Security Act of 2007 that phases out energy-wasting incandescent light bulbs and replaces them with 30% more energy-efficient alternatives like compact fluorescent light (CFL) bulbs.
The UL 1699 AFCI standard also includes qualification requirements for art fault detection with masking loads, which differ from resistive loads because masking loads mimic or short-out the spectral components that occur with arcing resistive loads. Masking loads can include switch-mode power supplies as used in computers, capacitor-start compressor motors as used in air-conditioning units, dimmer-controlled lighting, fluorescent lamps and vacuum cleaners. Since non-arcing masking load spectral components can hide or look like arcing resistive load spectral components, the UL 1699 standard requirements for masking loads indicate that prior art AFCI designs which use frequency-domain analysis to detect series arc faults are not commercially available for sale.
The present AFCI invention uses microprocessor-executed time-domain algorithms with an integrated method to detect parallel and series arc faults on linear load (resistive) and non-linear load (masking) current waveforms. While frequency-domain analysis cannot be used to detect series arc faults with masking loads, the present AFCI invention does use a frequency-domain algorithm to mitigate false arcing circuit detections and interruptions (nuisance tripping) due to normal arcing electric motor-driven appliances like ceiling fans and power drills. These appliances produce a significant 2nd harmonic spectral component compared to its 3rd harmonic, and thus, this spectrum is not consistent with the spectral components of an arc fault with resistive or masking loads.
As reflected by the limited number of commercial AFCI suppliers, many prior art AFCI designs have not successfully addressed the many realistic challenges in electrical circuit arc fault detection and interruption.
U.S. Pat. No. 7,864,492 (Restrepo et al.) discloses an illustration showing its embodiments use separate methods for detecting parallel and series arc faults, and discloses other illustrations showing how its zero crossing mask technique works on a linear load (resistive) arc fault condition. There is no disclosure of a method or technique to detect arcing conditions with a non-linear load (UL 1699 “masking load”). The present AFCI invention uses a microprocessor-executed integrated method to detect parallel and series arc faults on both resistive and masking load current waveforms.
U.S. Pat. No. 7,440,245 (Miller et al.) discloses a method of detecting an arc fault in a power circuit, said method comprising: determining a peak amplitude of a current pulse of a current flowing in said power circuit; determining whether the peak amplitude of said current pulse is greater than a predetermined magnitude; responsively employing at least one algorithm and said peak amplitude to determine whether an arc fault condition exists in said power circuit. Illustrations disclose that its method works on a linear load (resistive) arc fault condition. There is no disclosure of a method or technique to detect arcing conditions with a non-linear load (UL 1699 “masking load”). The present AFCI invention uses a microprocessor-executed integrated method to detect parallel and series arc faults on both resistive and masking load current waveforms.
U.S. Pat. No. 7,391,218 (Kojori et al.) discloses a method and apparatus to detect series and/or parallel arc faults in AC and DC systems, wherein a fundamental frequency component of the AC current signal is extracted using a discrete Fourier transform (DFT) algorithm and monitored for amplitude variation as a first arc detection measure, and non-stationary frequency-domain changes in the AC current signal as a second arc fault detection measure. The present AFCI invention does not use frequency-domain analysis for arc fault detection because that method does not work for detecting arc faults with masking loads.
U.S. Pat. No. 7,253,637 (Dvorak et al.) recognizes that frequency-domain analysis does not work for detecting series arc faults with masking loads, and discloses a circuit for determining whether arcing is present in an electrical circuit in response to an input sensor signal corresponding to current in said electrical circuit, which is in fact a “di/dt” sensor signal. The present AFCI invention uses a HECS integrated circuit that measures “i” (current) as the input sensing signal, not “di/dt” (change of current divided by change of time).
U.S. Pat. No. 7,062,388 (Rivers, Jr. et al.) discloses a frequency harmonic identifier for detecting series arcs on a power line that includes a frequency analyzer for providing harmonic content of a sensed current signal and decision logic for comparing a tested signal to at least one reference signal. The present AFCI invention does not use frequency-domain analysis for arc fault detection because that method does not work for detecting arc faults with masking loads.
U.S. Pat. No. 6,876,528 (Macbeth et al.) discloses a fault detector sensor that includes a current transformer, with two multi-turn windings each formed around a portion of the core, with one winding adjacent to each of the hot and neutral wires of the power line being protected. This prior art discloses a current transformer as the current sensing element, which is also used in many other prior art designs for electrical fault detection. The present AFCI invention uses a HECS integrated circuit, which is different from many prior art designs citing a current transformer as a “Hall sensor,” a “current sensor,” a “two-transformer resonant circuit,” a “clamp” or any other misused terminologies.
U.S. Pat. No. 6,751,528 (Dougherty et al.) discloses an electronically controlled circuit breaker comprising: a line current sensor sensing line current signals; a processor for determining the fundamental frequency of the current signals, wherein the processor processes a preselected number of multiples of the fundamental frequency, and squares and sums the multiples to yield even, odd, and fundamental values; even, odd, and fundamental bins within the processor for receiving the even, odd, and fundamental values, wherein the processor processes even arc signals and non-harmonic arc signals from the even, odd, and fundamental values in the bins; and, an expert arc algorithm within the processor having an accumulator for calculating an incremental value based on even arc signal and non-harmonic arc signal inputs. The present AFCI invention does not use frequency-domain analysis for arc fault detection because that method does not work for detecting arc faults with masking loads.
The system and integrated method of the present AFCI invention addresses a need in the art to not only meet, but exceed the UL 1699 AFCI standard requirements for detecting and interrupting electrical circuit parallel and series arc faults on both resistive and masking loads at current levels lower than 5 amps, and thus, protect property and save lives by preventing a major cause of electrical fires.